Mobile work platforms typically include a cage or platform that is designed to receive one or more human operators. The cage is mounted on a lift mechanism, such as a hydraulic boom or a scissor lift mechanism, that allows its height to be adjusted. The mobile work platform also includes a wheeled or tracked chassis, which allows it to be moved easily to a desired location. Various types of mobile work platform are available, including self-propelled, self-drive, trailer and vehicle-mounted platforms.
A load monitoring system is generally included to provide cage overload protection. Cage overload protection is required on mobile elevating work platforms as defined by European standard EN280:2001 Amendment 1. European standard EN954 is cited by EN 280 and describes the requirements for safety switches in electrical systems providing overload protection. EN954 Category 3 requires that common mode faults resulting from damage to the protection system are taken into account, if the probability of such a fault occurring is significant.
The main requirement of the aforesaid standards is that the load monitoring system monitors the static load and either provides an alarm or restricts operation of the work platform if the static load exceeds a predetermined limit. During operation of the work platform the dynamic load on the cage may increase or decrease depending on the acceleration of the operator cage. To avoid unintentional activation of the control system whenever the dynamic load exceeds the predetermined limit, the overload protection system is generally disabled during operation of the work platform. The static load safety limit is therefore set at a relatively low level to provide a safety margin that is sufficient to ensure that dynamic loads encountered during normal operation of the work platform do not jeopardise its stability.
Although the aforesaid safety standards provide a good degree of protection when the work platform is being operated correctly, dangers can still arise through careless or inattentive use. For example, when the work platform is being operated in the vicinity of an overhead obstruction, there is a danger that the operator platform may be driven into the obstruction by operation of either the lift mechanism or the drive mechanism for the wheels or tracks. This may endanger a human operator occupying the platform. A load monitoring system of the type described above would provide no protection in such a situation, as it would be disabled during operation of the lift mechanism or the drive mechanism.